I am looking at schematic of a Japanese SSB transceiver. It has a type of
balanced modulator I've never seen before. The audio from the mic is
amplified once by a conventional class A transistor amp, then fed into
an IC. The IC has a 5 MHz crystal. Based on the block diagram, the
purpose of the IC is to take 300-2700 Hz audio and shift it to 4400-2000
Hz, before it is fed into a balanced modulator using four diodes.
Does anyone have an idea why they would shift and invert the audio
passband before sending it to the balanced modulator? There are easier
ways of getting the opposite sideband (move the BFO crystal relative to
the filter) and I can't imagine why they'd want to shift the audio up,
except maybe to improve opposite sideband supression (it's a very simple
4 pole crystal ladder filter). Could the sideband inversion just be an
artifact of the intended frequency shift? One clue is they move the BFO
(by changing load on the bfo crystal) about 3800 Hz when going between TX
and RX modes. The audio processor chip I'm referring to is only in the TX
audio path - the RX audio path is completely conventional (it's a transceiver).
The chip doing the magic is a M64021, and I have no other specs on it.